The invention relates to an amplifier circuit comprising a control circuit, an amplifier stage connected thereto, two supply terminals coupled to supply sources for applying mutually different potentials to the supply terminals, the amplifier stage comprising 2n transistors, where n is an integer greater than one. Each of the transistors have a main current path and a control electrode, the main current paths being connected in series between the two supply terminals. The amplifier stage further comprises an output terminal connected to the junction of the nth and the (n+1)th main current paths of the series arrangement of transistors, the n transistors located between a first terminal of the two supply terminals and the output terminal being of a complementary conduction type to the n transistors located between the output terminal and the second terminal of the two supply terminals. 2(n-1) further supply terminals are also provided, each of which is connected via at least one diode to one junction point of consecutive main current paths of the series arrangement of the transistors, the junction points to which the further supply terminals are connected, not being connected to the output terminal. The further supply terminals are also connected to further supply sources for applying to the supply terminals supply voltages located between the potentials of the two supply sources, the diodes being connected such that the diode connected to a predetermined junction point is reverse-biassed when the potential of that junction point lies between the potential of the further supply terminal coupled to that diode and the potential of the supply terminal on the same side of the load connection as the further supply terminal.
Such an amplifier circuit, in which a sequence of supply potentials, which decrease incrementally from the first to the second supply terminal, are applied to the respective 2(n-1) further supply terminals, is disclosed in U.S. Pat. No. 3,622,899. Such an amplifier circuit may be used inter alia as an audio amplifier or as a driver stage in the line circuit of telephony systems.
In these amplifier circuits, the magnitudes of the instantaneous output voltage and the direction of the output current, determine which of the transistors of the series arrangement are conducting and which are non-conducting. As the value of the supply voltage is determined by the number of transistors which are conducting, this has the advantage that, in these amplifiers, a wide voltage range of the output signal is accompanied by a low internal dissipation.
However, such an amplifier circuit is not suitable for operation with output voltages which are more than twice the collector-emitter breakdown voltage of the transistors used. This prevents, in particular, the use of such amplifier circuits in integrated form in driver stages in subscriber's circuits or audio and video amplifier applications where high operating voltages are used.
Furthermore, the reduction of dissipation is small only when the sign of the output current is opposite to the sign of the output voltage, as may be the case during a portion of each period of the output signal as the result of a reactive load or when there are large common mode signals on subscriber's lines in telephony systems.